In other words, I'm guessing based on past experience and my current experiences in the game, having seen the AI operate on their Beginner leveled Masters and one Competent Master.
There's no reason to be rude V_V

That doesn't even make sense.

Maybe my question was not clear, so here it is, clarified further >

Start a single player scenario/campaign
Choose difficulty level for YOUR hero, say, Beginner => this should make the game easier for you, the human player, and THIS IS CLEAR. But you can also change the difficulties for the AI players, from Beginner to Overlord. What does that actually mean?

Hopefully, this will help: http://www.gog.com/forum/eador_genesis/game_hints_and_tips/post4

Indeed, that's exactly the answer. Thank you.

Interesting. So basically for difficulty levels for humans and AI works the opposite way. So if I choose beginner level for me things will be easy for me. But if I choose beginner for AI it means things are difficult for the AI because AI will start with low resources, they will receive low experience in battle, and they will face strong neutral enemies right?
So basically that's why I'm winning easily (in skirmish I chose experienced level for both me and the AI), thinking that in that way nobody would have had advantages.
So to be "fair" I should choose expert level, so everyone starts with the same amount of money, and faces neutral enemies with the same amount of life). Right?

Yes. Exactly. But be prepared for really hard struggle ;)

I like struggling :P When you win it's a great satisfaction! I thought at "Experienced" the AI was facing easier neutral enemies like me, instead I was having a double advantage over others AI players! I will start a new match. It will be much more difficult, but more challenging. Moreover after my actual skirmish match now I am more experienced too :)

The warning has been given... >:D

I love a good challenge, in fact it's one of the things that (for me) makes a game worth playing.
I've a policy of turning up the difficulty to max (and enabling 'hardcore mode' if the game has one) on every new game and never looking back. Been doing this almost exclusively for a few years now. I'm happy to say that in Eador I've found a game which doesn't require me to play max difficulty to find a challenge, in fact I think I'll be working on mastering this game for quite some time but just as legionarius769 above me, I think that difficulty is all to the good. :)